GPS receivers can be characterized by performance criteria such as acquisition and tracking time, which reflect the amount of processing necessary to detect and lock on to GPS satellite signals and hence the amount of time needed to begin accurately reporting a user's position. The acquisition time, tracking ability, sensitivity, position accuracy and other performance parameters of GPS receivers can be affected by a variety of factors. Those factors include the precision with which frequency references for radio frequency detection and other purposes can be generated and managed within the device. L1 GPS signals used for civilian coarse acquisition (C/A) purposes are broadcast at 1.575 GHz from the associated NAVSTAR satellites. Russian GLONASS satellites broadcast in a similar frequency range.
Handheld, vehicle-mounted, stationary and other positioning receivers including GPS receivers require frequency stability in their clocks generally in the range of a few parts per million or less to accurately derive Doppler and other data from reference signals, and therefore triangulate a precise receiver position within a reasonable acquisition time.
Recently, market trends have developed toward GPS functionality combined with other communications services. Various wireless devices, such as cellular telephones, digital pagers, wireless personal digital assistants, 802.11a/b/g and other clients may all be combined with GPS location receivers for various applications.
However, the accuracy of reference clocks generally employed in cellular telephones and other communications devices may generally not be as great as that needed for useful GPS service, which as noted may require accuracy to within at least a few parts per million, down to tenths of 1 part per million or less for increased performance in acquisition time, tracking, sensitivity, position accuracy and other factors. Cellular telephones on the other hand may contain uncompensated oscillators accurate to within only perhaps five to tens of parts per million, depending on implementation. Cellular devices may tolerate higher frequency variability in part because handsets or other devices may be able to derive a stable frequency reference from a base station or the wireless network, itself.
In the case of a GPS receiver combined with a cellular telephone for caller location service as mandated by the Federal Communications Commission, a cellular telephone's local crystal oscillator, tuned to 16.8 MHz or another base frequency, may for instance have a frequency variance of ±30 ppm or more or less. A cellular handset's internal clock may therefore not be sufficient to drive GPS circuitry in a combined device for useful GPS operation by itself. Temperature compensation circuits operating on ordinary crystal oscillators may improve the frequency reference to perhaps ±5 ppm or so, although those types of parts may add to the cost of a relatively low-cost mobile device. Solutions such as supplying two corrected reference oscillators, one for GPS and one for cellular or other communications service at different frequencies, for instance, may not be economical in a combined device. Other problems exist.